First things to try after you've mastered basic static web site creation.
For now you should focus on getting really confortable creating your own site with the static web server environment described above, but eventually you may want to get more advanced. Here are some suggestions on what to do and learn next.
If you have not done these Khan Academy tutorials yet then do these before continuing:
Read over, or at least poke around in, the reference documentation to see just how many things you can do with HTML and CSS:
At some point you'll probably want to know more about how the Internet works. Consider watching the Internet 101 lessons on Khan Academy. This is useful because it will help you understand some of the more advanced things described below, and some things mentioned above such as domain names.
KISSr limits your usage to 1,000 page views per month with the free account. While you can pay them $5/month to remove this restriction and gain the ability to create as many subdomains as you want, you might want to try using GitPages which doesn't have the usage limitations.
GitPages is slightly harder to use because you will need to learn to use Git to upload your files. However, the text editors I mentioned come with Git extensions that will allow you to push updates from the text editor so it's easier to do once you set it up.
While you can just upload files to a Git project using your web browser, learning Git is worthwhile because this knowledge will come in very handy for the more advanced server hosting environments and for programming in general. At first, at least learn to use the GUI Git client provided by GitHub.
GitPages has the limitation that you can only have one domain name per user and it has to be your username. Also, everything you upload there is public to the world unless you pay a subscription fee. So be warned about that. You can use your own domain name with GitPages.